1. Field of the Invention
The present invention relates to a device for use in user stations of a CAN bus system which is equipped to provide and process a global time information in the event-controlled exchange of data with other user stations of the CAN bus system, as well as the bus system developed using such user stations.
2. Description of the Related Art
From International standards ISO 11898-1, -2 and -3 the “Controller Area Network” (CAN) is known, which includes an event-controlled method for the exchange of data between users of a bus system. The media access control method is based on the bit-wise arbitration, in which a plurality of user stations are simultaneously able to try to transmit data via the channel of the bus system, without thereby interfering with the data transmission. The user stations are able to ascertain the logical state (0 or 1) of the channel while transmitting a bit over the channel. If a value of the bit sent does not correspond to the ascertained logical state of the channel, the user station terminates the access to the channel.
Because of the bit-wise arbitration, a non-destructive transmission of the data frame over the channel is achieved. Good real time properties of the CAN are yielded thereby.
From international Standard ISO 11898-4, for example, a extension of the CAN is known that is designated as “Time-Triggered CAN” (TTCAN). According to the TTCAN protocol specification, in the so-called system matrix, a time window structure is defined which has a plurality of successive base cycles which, in turn, are made up of time windows (often also designated as “time slots”) whose sequence repeats regularly. In this way, each message, and consequently the sending user station, is able to have allocated to it a certain time window, within which this message has to be transmitted. There is also the possibility, in time windows, of releasing the access, coordinated by arbitration, to a plurality of user stations. In TTCAN, the access to the channel is thus at least partially coordinated by a time-controlled method.
In TTCAN, a distinction is made between TTCAN Level 1 and TTCAN Level 2. In both cases, the communication takes place in a sequence of base cycles which are initiated in each case by a reference message. The difference between the two levels is that in Level 2 the reference message includes information on synchronizing a global time information or global time. Thus, TTCAN is distinguished in that the user stations may be provided with global time information by a time master. One specified procedure for synchronization takes care that all user stations use uniform global time data.
CAN controllers, which control TTCAN Level 2, have a circuit (counter, arithmetic unit, control unit) which synchronizes and calibrates the Global Time in Hardware, and enables time measurement and time status indication.
The TTCAN protocol also includes monitoring functions, which are supposed to ensure that the CAN communication maintains the previously specified fixed time reference. If errors occur in the time reference, the time base is indicated as being invalid. In response to serious errors, the CAN communication may be stopped.
In certain applications, the time control of the bus communication is either not possible or at least not meaningful, or the effort to set up the system matrix is unjustifiably costly. Still, providing global time information, as is performed by TTCAN Level 2, is desirable in order to be able to use the time information in the algorithms which run in the user stations.
The methods known from the related art do not deliver results that are satisfactory in every respect.